Three operant analysis studies of pre-linguistic development will be undertaken with infants 8 to 14 months of age. Each study will consist of two experiments. The first study will be a systematic replication of the Hursh and Sherman (1973) experimental analysis of the effects of social reinforcement and modeling on specific infant vocalizations, with infants 12 months of age on their first session, with developmentally-delayed, as well as normally- developing, infants, and with assessment of generalized imitation. Although "imitation" in infants has been described as early as 12 to 21 days of age, generalized imitation (Baer, Peterson & Sherman, 1967; Gewirtz & Stingle, 1968) has not been demonstrated in children under three years of age. Furthermore, in the first study, an analysis of the effects of reinforcement contingencies will be undertaken with a brief reversal period in which infants will be praised for responding differently from the model. In the second study, to be able to accomplish a downward extension of these procedures to infants as young as 8 months of age, who do not show vocal imitation, gestural imitation will be examined to determine whether similar procedures are effective for producing both vocal and gestural generalized imitation. Furthermore, as in the first study, an analysis of the effects of reinforcement contingencies will be undertaken with a brief reversal period in which infants will be praised for responding differently from the model. These procedures will be replicated with infants who are developmentally delayed. Finally, in the third study, the focus will shift to an analysis of variables other than reinforcement that affect the degree or kind of response generalization obtained. Specifically, the third study will be an examination of the effects of the different vocal and gestural response topographies on nonreinforced probe responding in the generalized imitation paradigm. Variables influencing response generalization will be examined with developmentally-delayed infants as well.